The present invention relates generally to television receivers of the type having VHF and UHF tuners and, more particularly, concerns a novel switching circuit for coupling the VHF and UHF tuners of a television receiver to a single antenna input terminal.
Television receivers conventionally include both VHF and UHF tuners, each adapted for tuning a respective band of television signal frequencies. The VHF tuner, for example, is typically designed for tuning the VHF television signal frequencies including the low-VHF band from 54 to 88 MHz and the high VHF band from 174 to 213 Mhz. In addition, many current VHF tuners are also adapted for tuning portions of the cable television (CATV) signal frequency spectrum including the CATV mid-band from 100 to 174 MHz, the CATV superband from 216 to 300 MHz and the CATV hyperband from 300 to nearly 400 MHz. The UHF tuner is typically designed for tuning the UHF television signal frequency spectrum between 470 and 890 MHz.
Television receivers having both VHF and UHF tuners have conventionally been designed with two separate antenna input terminals, a VHF input terminal for coupling the VHF/CATV band television signal frequencies to the VHF tuner and a UHF antenna input terminal for coupling the UHF band of television signal frequencies to the UHF tuner. In the case where separate VHF and UHF antennas are used to receive broadcast VHF and UHF television signals, the VHF antenna is connected to the VHF antenna input terminal and the UHF antenna is connected to the UHF antenna input terminal. On the other hand, in the case where a single antenna is used to receive both VHF and UHF television signals, a diplexer is typically connected between the antenna and the television receiver to separate the received VHF and UHF signals for application to the respective antenna input terminals of the receiver. A conventional prior art diplexer of the foregoing type is shown in FIG. 1. The diplexer includes an input terminal 10 coupled to a low-pass filter section 12 and a high-pass filter section 14. The low-pass filter section 12 of the diplexer exhibits a frequency response as represented by curve 16 of FIG. 2 for passing the received VHF signal frequencies to the VHF tuner and the high-pass section 14 exhibits a frequency response as represented by curve 18 for passing the received UHF signal frequencies to the UHF tuner.
It would be desirable to avoid the use of diplexers by providing a television receiver with a single antenna input terminal for connection directly to a combination VHF/UHF antenna and separating the VHF and UHF signal frequencies internally within the receiver for application to the VHF and UHF tuners. Due to the ever increasing encroachment of the upper end of the CATV frequency spectrum on the lower end of the UHF frequency spectrum, to a point where little or no frequency separation exists therebetween, the use of such a single antenna input terminal would provide the added advantage that the UHF tuner could be used to tune the higher frequency CATV channels thereby allowing for the channel capacity of the CATV system to be greatly increased.
To make the use of a television receiver in the foregoing manner practicable a suitable diplexing circuit must be provided between the single antenna input terminal and the VHF and UHF tuners of the receiver to appropriately separate the broadcast signals, whether received over-the-air or through a cable. One approach to providing such a diplexing circuit would be to use a conventional diplexer of the type illustrated in FIG. 1 to selectively couple the received television signals to the VHF and UHF tuners. With reference to FIG. 2, it will be noted that the frequency response characteristics of the prior art diplexer of FIG. 1 exhibit a considerable overlap between the low and high-pass curves 16 and 18 in the region of the frequency spectrum identified by reference numeral 20. This overlap in the response characteristics of the filter sections 12 and 14 poses no problem when the diplexer is used as intended since no television signals are broadcast over-the-air in frequency spectrum region 20. However, the overlapping response characteristics would prevent the diplexer from being used as contemplated herein since received CATV signals in the frequency spectrum region 20 would be subject to excessive insertion losses upon being coupled to either the VHF or UHF tuner.
The insertion loss problem characterizing the passive splitter design described above can be eliminated to some extent through the use of an active switching circuit as illustrated in FIG. 3. In this switching circuit, an active switching element 22, e.g. a switching diode, is connected between the single antenna input terminal 10 and the UHF tuner and a series of three switching elements 24, 26 and 28, which may also be switching diodes, are connected between antenna input terminal 10 and the VHF tuner. In the VHF mode, a U/V bandswitch control signal generated by the channel selector (not shown) causes switching elements 24 and 28 to assume a conductive state and switching elements 22 and 26 a non-conductive state as shown. As a consequence, substantially all of the signals received at input terminal 10 are coupled to the VHF tuner which tunes the particular channel designated by the channel selector. It has been found preferable to incorporate a 54 MHz high-pass filter 30 between antenna input terminal 10 and the switching circuit to protect the switching elements from strong out-of-band interference such as amateur radio transmissions and low frequency signal spikes caused by lightning. In the UHF mode of operation, the U/V bandswitch control signal generated by the channel selector causes switching elements 22 and 26 to assume a conductive state and switching elements 24 and 28 a non-conducting state. In this configuration, substantially all of the received signals are coupled to the UHF tuner which tunes the particular channel designated by the channel selector, with the switching elements 26 and 28 serving to provide additional necessary isolation from the VHF tuner. While the foregoing switching circuit is believed to provide adequate performance, it requires the use of a relatively complex and costly circuit, including, for example, four switching diodes and a high-pass filter, and is therefore not especially suited for use in mass produced consumer electronic products.
It is therefore an object of the present invention to provide an improved switchable diplexing circuit selectively operable for coupling received television signals from a single antenna input terminal of a television receiver to either the VHF or UHF tuner of the receiver without excessive insertion loss.
It is a further object of the invention to provide a switchable diplexing circuit of the foregoing type which is relatively inexpensive, preferably incorporating no more than two active switching elements while providing adequate isolation between the VHF and UHF tuners of the receiver.